1. Field of the Invention
The present invention relates generally to a solid state image sensor using a CCD (charge coupled device), and is directed more particularly to a solid state image sensor to reduce the generation of picture defects (grey vertical lines in a displayed image) which are caused by such a case where an over-flow drain area, which will absorb and remove an excess charge, is provided so as to suppress a blooming phenomenon at the image portion thereof.
2. Description of the Prior Art
In a CCD solid state image sensor of the frame transfer system, if its image portion is formed of a buried channel type, there appears a blooming phenomenon for an intensive light impinging thereon. To avoid this defect, adjacent a channel area, serving also as a light receiving area, along each vertical line in the image portion, provided are an over-flow control gate area and an over-flow drain area to absorb an excess charge, which is generated when the intensive light is received, to the over-flow drain area passed through the potential barrier of the over-flow control gate area. At this time, a potential, i.e. potential 1 of the over-flow control gate area is set between a potential 2 at a storage area in the channel area and a potential 3 at a transfer gate area as shown in the graph of FIG. 1. However, a potential difference .0..sub.c thereof (difference between potentials A and B i.e. maximum handling charge) is constant without being dependent on a gate voltage V.sub.G applied to a gate electrode (a so-called transfer electrode). In other words, this means that as the image portion the maximum handling charge during the light receiving period becomes equal to the maximum handling charge during the charge transfer period.
Now, let it be assumed that, as shown in FIG. 2, the maximum handling charge at each of picture elements 4 of the image portion all have the capacity of "10" and the maximum handling charge of each of picture elements corresponding to a stage [4] and at a channel area B has only the capacity of "9" due to some reasons. Thus, during the light receiving mode, the stages [1], [2] and [3] of the channel B each can store the charge amount of "10" but only the stage [4] stores the charge amount of "9". Since, however, upon charge transfer mode, in the stage [4] of the channel area B only the charge amount of "9" is handled without depending on the gate voltage as shown in FIG. 1, if the charge transfer direction is taken as indicated by an arrow y in FIG. 2, the charge amounts at the stages [1], [2] and [3] are each suppressed as the charge amount of "9" when they pass through the stage [4]. As a result, after the stage [4] of the channel B, the charges become less than the inherent charges or so-called defects of grey vertical lines appears on a picture screen.